1. Field of the Invention
The instant invention concerns peroxides and the decomposition of same to facilitate initiation of certain chemical reactions. More specifically, the invention contemplates the use of select heteropoly acids, or HPAs, to obtain relatively controlled decomposition of organic peroxides, especially peroxyketals and dialkyl peroxides. Importantly, the practice of the present invention allows for less reactive peroxides--which are correspondingly safer to handle and store--to be effectively substituted for more reactive peroxides which are proportionately more hazardous and difficult to use.
2. Description of the Prior Art
Organic peroxides have multifarious commercial application, the more important of which predominate in the polymer industry. There, organic peroxides are used inter alia to initiate the polymerization and copolymerization of vinyl and diene monomers such as vinyl chloride, styrene, ethylene, acrylic acid and esters, methacrylic acid and esters, vinyl acetate, acrylonitrile and butadiene. Additionally, they are used to cure or cross-link various resins such as unsaturated polyesters, including e.g. unsaturated polyester-styrene blends; vinyl esters, including e.g. terpolymers of ethylene-vinyl acetate copolymer as well as other elastomers, including e.g. ethylene-propylene copolymers; thermoplastics, including e.g. polyethylene; and rubbers, including e.g. silicone rubber and styrene-butadiene rubber.
It is generally believed that organic peroxides owe their efficacy in this regard to their ability to generate free radicals on cleavage of the oxygen--oxygen bond which characterizes the peroxide functional group. In the ordinary industrial course, cleavage is obtained thermally; that is, the peroxide is heated to a point where it decomposes to form oxygen-centered free radicals. The corresponding rate of thermal decomposition is principally affected by the structure of the peroxide as well as the conditions under which heating occurs. Because the rate of decomposition is a critical process consideration in the polymer arts, certain conventions have evolved to express the differences in rate that inhere between various peroxides.
One of the more common of these is the measure of thermal activity of a peroxide in terms of its 10-hour half-life temperature, or 10-h HLT. This is the temperature required to attain 50% decomposition of a peroxide in a period of 10 hours. Generally, this temperature can range from well below 20.degree. C. to well in excess of 200.degree. C. For practical purposes, the measure is a comparative one: of any two peroxides, the one with the lower 10-h HLT is the one more easily decomposed on heating.
Other parameters useful for assessing the thermal lability of a particular peroxide include the onset temperature of decomposition and the self-accelerating decomposition temperature (SADT). Onset temperature is the point at which an uncontrolled thermal decomposition starts. Typically, it is measured by differential thermal analysis (DTA) using a small sample of peroxide in a sealed cell tested to determine the point at which the rate of temperature increase in same exceeds a pre-determined value. SADT, which can be directly correlated with onset temperature insofar as it increases linearly with onset temperature, represents the lowest temperature at which a given peroxide of a specific sample size undergoes self-accelerating decomposition within a set period of time, usually seven days. While these tests have especial utility in determining how a peroxide should be handled, stored and transported, they also give guidance in selecting a peroxide for a particular end use.
From a safety standpoint, it is desirable to use peroxides that are more thermally stable, e.g. those that have higher onset temperatures of decomposition, since these are less subject to unintended decompositions--which can be violently explosive--and require commensurately less precautions in handling, which can otherwise encumber processing. Paradoxically, it is this stability which makes these same peroxides less desirable as initiators or curing agents since higher temperatures have to be implemented to achieve the very reactivity for which they are being employed. The use of higher temperatures creates its own complications and hazards, and makes overall operations less economically efficient.
Various attempts have been made to reconcile these contradictory considerations thereby enabling the use of safer peroxides in a manner that mimics not only the reactivity of those that are more unstable, but does so under comparable conditions of temperature and cost. The most widely hailed in this regard involves the use of promoters, also known as activators, accelerators or destabilizers. These materials significantly decrease the amount of energy needed to break the oxygen--oxygen bond, thus lowering the onset temperature and accelerating the rate of peroxide decomposition. The enhanced decomposition that results usually occurs at temperatures well below that required under normal circumstances.
Promoters typically utilized in the polymer industry generally fall into two categories: those that contain transition metal salts, including metals such as cobalt, manganese, vanadium, copper, chromium, titanium, iron and the like; and those that contain tertiary amines. An example of the first are cobalt carboxylate promoters such as cobalt naphthenate which are commonly employed to destabilize ketone peroxides and cumene hydroperoxide which is often used to initiate the cure of unsaturated polyester and vinyl ester resins in the fiber reinforced polymer industry. Exemplifying the second are N,N-dimethylaniline, N,N-diethylaniline and N,N-dimethyl-p-toluidine--all widely used in the destabilization of benzoyl peroxide.
These known promoters do enhance peroxide reactivity, but with a cost: The transition metal-based materials tend to discolor the accelerated resins; thus cobalt-based promoters are known to tint cured polymers from pale to dark pink, and even to a blue green color. Amine accelerated systems are known to impart a disagreeable odor to the resultant polymer, which then also has a tendency to yellow with age. And both classes of promoters are known to cause significant and undesirable exotherms when applied in certain situations. Finally, not all classes of organic peroxides, such as perketals and peroxymonocarbonates, can be effectively promoted by conventional accelerators.
There is accordingly an ongoing need for a peroxide promoter that effectively and efficiently increases the reactivity of safer, high temperature organic peroxides so that they can be used in lieu of more reactive and hazardous peroxides, which promoter also does not evince the discoloration and malodorousness that attends the use of promoters known heretofore.
Heteropoly acids are thermally robust materials (up to 180.degree.-300.degree. C.) that are also stable against oxidation. These materials in the main are used in solid state catalysis, including catalytic oxidation. They are also known to be initiators of certain cationic polymerizations, including polymerizations of cyclic ethers and acetals. Other attempts to use these materials in the polymerization of olefins, and, separately, the oligomerization of unsaturated hydrocarbons have also been reported.